Distributed antenna system for MIMO signals

ABSTRACT

A distributed antenna system includes a multiple-input and multiple-output (MIMO) base station configured to output at least a first signal and a second signal. At least one master unit communicates with the MIMO base station. At least one remote unit communicates with the master unit. At least one antenna is coupled with the remote unit for receiving signals from the remote unit. A hybrid coupler is coupled between the remote unit and antenna and is configured to receive the first signal Ch1 and the second signal Ch2 from the remote unit on respective first and second ports and to provide an output signal on at least one output port. The output signal includes at least a portion of the first signal and at least a portion of the second signal. The antenna is coupled with the output port.

RELATED APPLICATIONS

This Application is a continuation-in-part of U.S. patent application Ser. No. 13/794,416, filed Mar. 11, 2013, entitled, “DISTRIBUTED ANTENNA SYSTEM FOR MIMO SIGNALS”. U.S. patent application Ser. No. 13/794,416 is a continuation-in-part application of U.S. patent application Ser. No. 12/634,212, filed Dec. 9, 2009, entitled “DISTRIBUTED ANTENNA SYSTEM FOR MIMO SIGNALS”, now issued U.S. Pat. No. 8,396,368, issued Mar. 12, 2013. U.S. patent application Ser. No. 13/794,416 is also a continuation of International Application No. PCT/US2011/062640, filed Nov. 30, 2011, entitled “DISTRIBUTED ANTENNA SYSTEM FOR MIMO SIGNALS”, which International Application in turn, claims priority to Italian Patent Application No. 2010A000714, filed Dec. 1, 2010, entitled “DISTRIBUTED ANTENNA SYSTEM FOR MIMO SIGNALS”. The applications and disclosures are all hereby incorporated by reference in their entireties as though fully disclosed herein.

FIELD OF THE INVENTION

Embodiments of the invention are directed to wireless communication systems, and specifically directed to a distributed antenna system for a wireless MIMO communications.

BACKGROUND OF THE INVENTION

A contemporary wireless communication system, such as distributed antenna system 10, is shown in FIG. 1, and includes a number of remote units 12 distributed to provide coverage within a service area of the system 10. In particular, each remote antenna unit 12 typically includes an antenna 14 and suitable electronics. Each remote unit is coupled to a master unit 16. Each master unit 16 is, in turn, coupled to a RF combination network 18 that combines the signals from at least one single-input- and single-output (“SISO”) base transceiver station (“BTS,” or more simply, “base station”) 20 (hereinafter, “SISO BTS” 20). The system 10 may further include a system controller 22 to control the operation of each master unit 16. As illustrated in FIG. 1, the system 10 may include a plurality of master units 16 and a plurality of SISO BTSs 20, each master unit 16 configured to provide a combination of the signals from at least two SISO BTSs 20 to its respective remote units 12.

As illustrated in FIG. 1, each remote unit 12 may broadcast a wireless signal 24 that, in turn, may be received by a wireless device 26 that may be a mobile device, such as a telephone device or a computing device. In particular, and as discussed above, the wireless signal 24 from each remote unit 12 may be a combination of signals from the at least two SISO BTSs 20. Thus, the wireless device 26 may communicate with the system 10 through any of the wireless signals 24 from the remote units 12.

To improve wireless communications, such as communications from a base station to mobile devices, Multiple-Input/Multiple-Output (“MIMO”) technology might be utilized to provide advanced solutions for performance enhancement and broadband wireless communication systems. Through various information series studies, it has been shown that substantial improvements may be realized utilizing a MIMO technique with respect to the traditional SISO systems. MIMO systems have capabilities that allow them to fully exploit the multi-path richness of a wireless channel. This is in contrast with traditional techniques that try to counteract multi-path effects rather than embrace them. MIMO systems generally rely upon multi-element antennas at both of the ends of the communication links, such as in the base station and also in the mobile device. In addition to desirable beam-forming and diversity characteristics, MIMO systems also may provide multiplexing gain, which allows multi data streams to be transmitted over spatially-independent parallel sub-channels. This may lead to a significant increase in the system capacity. Generally, the systems illustrated in FIG. 1 cannot take advantage of MIMO technology.

For example, the wireless device 26 of FIG. 1 communicates with only one of the remote units 12, though it may be in the range of a plurality of remote units 12. The wireless signals 24 from each remote unit are typically at the same frequency and carry the same data, and communication between a plurality of remote units 12 and the wireless device 26 simultaneously may result in signal degradation and collisions. Moreover, data bandwidth from the wireless device 26 is constricted to the speed of reception and processing of data from one remote unit 12.

It is therefore, desirable to take advantage of MIMO signals within a wireless system, such as distributed antenna system, without requiring an entirely new system to be installed for handling MIMO signals.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a distributed antenna system (“DAS”) and methods of use that can be used to provide a multiple-input and multiple-output (“MIMO”) mode of operation. In particular, some embodiments include a MIMO base station as a signal source configured to output at least a first MIMO signal and a second MIMO signal. A hybrid coupler is coupled to receive the first and second signals. The coupler is configured to receive the first MIMO signal and the second MIMO signal on respective first and second ports and provide an output signal on output ports. The output signals include at least a portion of the first signal and at least a portion of the second signal. The system further includes at least one master unit and at least one remote unit communicating with the master unit and configured to communicate at least the MIMO output signals to a device, such as a customer's wireless device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a contemporary distributed antenna system.

FIG. 2A is a block diagram of a distributed antenna system consistent with embodiments of the invention.

FIG. 2B is a block diagram of a distributed antenna system consistent with embodiments of the invention.

FIG. 3 is a block diagram of a distributed antenna system consistent with embodiments of the invention used with an indoor environment.

FIG. 4 is a detailed block diagram of a master unit utilized in embodiments of the invention.

FIGS. 5A and 5B are a detailed block diagram of a portion of a remote unit utilized in embodiments of the invention.

FIG. 6 is a detailed block diagram of an alternate portion of a remote unit utilized in embodiments of the invention.

FIG. 7A is a block diagram of MIMO BTS in an outdoor scenario.

FIG. 7B is a block diagram of an alternative distributed antenna system consistent with embodiments of the invention.

FIG. 8 is a block diagram of a 90° 3 dB hybrid coupler and transfer function representation.

FIG. 9 is a block diagram of an overview of LTE physical channel processing.

FIG. 10 is a distributed antenna system for implementing the invention.

FIG. 11 is a block diagram of an alternative distributed antenna system consistent with embodiments of the invention.

FIG. 12 is a block diagram of an alternative distributed antenna system consistent with embodiments of the invention.

FIG. 13 is a block diagram of an alternative distributed antenna system consistent with embodiments of the invention.

FIG. 13A is a block diagram of an alternative distributed antenna system consistent with embodiments of the invention.

FIG. 14 is a block diagram of an alternative distributed antenna system consistent with embodiments of the invention.

FIG. 15 is a block diagram of an alternative distributed antenna system consistent with embodiments of the invention.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of embodiments of the invention. The specific design features of the system and/or sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments may have been enlarged, distorted or otherwise rendered differently relative to others to facilitate visualization and clear understanding.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2A illustrates a schematic view of one possible implementation of a MIMO system, wherein a MIMO base station is incorporated with a distributed antenna system, such as that shown in FIG. 1. With respect to FIG. 1, like reference numerals are utilized in FIG. 2A where applicable. As illustrated, two SISO base stations 20 (each SISO base station 20 a “SISO BTS” 20) are coupled with each of the remote units. Additionally, a MIMO base station 30, including antennas 31 and 32, are coupled with the remote units 12. Antenna 1 of the MIMO BTS 30 is coupled with remote units 12 a and 12 d through a first master unit 16 (MASTER UNIT 1). Antenna 2 of the MIMO BTS 30 is coupled with remote units 12 b and 12 c through as second master unit 16 (MASTER UNIT 2). As such, as illustrated by the wireless signals 24 produced at each remote unit, each master unit will transmit signals from the MIMO BTS 30, in addition to a combined signal from the combination of signals output by the SISO BTSs 20. However, because each antenna is not coupled to all the remote units, each remote unit will only transmit one of the two available MIMO signals as shown. The respective wave fronts are illustrated corresponding to the feed or connection lines from each of the appropriate antennas 31, 32 in FIG. 2A.

While the embodiment illustrated in FIG. 2A may be utilized to provide the availability of MIMO signals within a distributed antenna system, such a system may not realize all of the desired performance improvements associated with a MIMO system. For example, even if wireless device 26 receives all the MIMO signals from a combination of at least two of the remote units, there may be a received RF power imbalance because the wireless device 26 might be located much closer to one remote unit 12 than to another. Furthermore, accordingly to wireless standards that support MIMO features, there are some signaling parameters, such as the WiMAX Frame Preamble or the LTE Primary Synchronization Signal (“P-SS”), which are, or actually can be, transmitted by only one of the MIMO BTS antennas 31, 32. Therefore, in a MIMO system, as in FIG. 2A, wherein these signals are not transmitted by all of the remote units, the system may not be sufficiently reliable unless there is a very high level of coverage redundancy/overlap between the remote units. In operation, dynamic switching between SISO and MIMO operating modes may present performance problems.

Furthermore, it will be appreciated that, in some embodiments, the first and second signals from the MIMO BTS 58 may be separately provided to respective summing circuits 48 a-b and/or master units 46 a-b rather than passing through hybrid coupler 58, such as the embodiment shown in FIG. 2A.

In some embodiments, the summing circuit 48 a is configured to provide a first master unit signal 72 that is a combination of the combined SISO BTS signal 56 a and the first combined MIMO signal 68. Summing circuit 48 b is configured to provide a second master unit signal 74 that is a combination of the combined SISO BTS signal 56 b and the second combined MIMO signal 70. The master units 46 a-b and remote units 42 a-h, in turn, may be controlled by a system controller 76, which may provide overall supervision and control of the master units 46 a-b and remote units 42 a-h, as well as alarm forwarding.

In some embodiments, each remote unit 42 a-h may be connected to their respective master units 46 a-b via high speed digital transport mediums or links 80 a-b, 82, 84 a-b and/or 86 a-b. Alternatively, an analog transport medium/link might be used for connecting the remote units with respective master units. Also, the transport links might be implemented as optical links using optical fiber as discussed below. With such fiber, the traffic between the remote units and master units might be implemented using a radio-over-fiber (RoF) format. In this manner, the first master unit signal 72 and/or the second master unit signal 74 are provided to at least a portion of the remote units 42 a-h in a digital format, which may assist in preventing at least some degradation due to transmission line effects. It will be appreciated by one having ordinary skill in the art that filtering may also be used to allow and/or prevent the distribution of specific signals. As such, and in some embodiments, each of the links 80 a-b, 82, 84 a-b and/or 86 a-b may be a wideband digitally modulated optical interface, such as fiber optic cable. Thus, each master unit 46 a and/or 46 b may be configured to digitize their respective master unit signals 72 and/or 74 and output those digital signals for their respective remote units 42 a-42 d and/or 42 e-h. These digital output signals may, in some embodiments, be time division multiplexed into frames and converted into a serial stream. The remote units 42 a-42 d and/or 42 e-h, in turn, may be configured to receive the digital output signals from their respective master units 46 a and/or 46 b, convert the digital output signals into electrical signals, if necessary de-frame various time slots and/or de-serialize the electrical signals, and transmit the electrical signals via their local antenna 44 to at least one wireless unit 90.

The remote units 42 a-h are configured to send and/or receive digital RF voice and/or data signals to and/or from a wireless unit 90 via their local antennas 44. As discussed below, depending on how the remote units are coupled to the master units, the remote units 42 b, 42 d and/or 42 f may also be configured to receive a digital signal from remote units 42 a, 42 c and/or 42 e, respectively, which precede it in a chain. This digital signal between remote units may contain signals from the wireless unit 90 received by the preceding remote units 42 a, 42 c and/or 42 e. The digital signal may then be combined with another signal received by the remote units 42 a, 42 c and/or 42 e. As such, digital wireless signals from wireless units 90 may be combined and/or transmitted back to a respective master unit 46 a and/or 46 b. The master units 46 a and/or 46 b may then convert a signal from its respective remote units 42 a-d and/or 42 e-h from an optical signal to an electrical signal and send the electrical signal to the SISO BTSs 54 a-b and MIMO BTS 58, which may be configured to detect and receive their respective portions thereof. Alternatively, the master units 46 a and/or 46 b may then convert a signal from its respective remote units 42 a-d and/or 42 e-h from an optical signal to an electrical signal, separate the electrical signal into a plurality of electrical signals in a plurality of bands corresponding to those utilized by the SISO BTSs 54 a-b and MIMO BTS 58, convert the plurality of electrical signals into a plurality of analog signals, and send the plurality of analog signals to the SISO BTSs 54 a-b and/or MIMO BTS 58.

As illustrated in FIG. 2B, by way of example, a master unit 46 a-b may be selectively connected to respective remote units 42 a-h in a number of ways. For example, master unit 46 a is illustrated as connected to remote units 42 a-b through half-duplex link 80 a for uplink to the remote units 42 a-b and half-duplex link 80 b for downlink. However, master unit 46 a is illustrated as connected to remote units 42 c-d through full-duplex link 82. Similarly, master unit 46 b is illustrated as connected to remote units 42 e-f through half-duplex link 84 a for uplink to the remote units 42 e-f and half-duplex link 84 b for downlink. However, master unit 46 b is illustrated as connected to remote unit 42 g through full-duplex link 86 a and connected to remote unit 42 h through full-duplex link 86 b. As such, in a full-duplex link, the uplink signals and downlink signals are carried on different wavelengths and a wavelength division multiplexer (“WDM”) is employed to combine and/or split the two optical signals at the master units 46 a-b and remote units 42 a-h. Alternatively, the master units 46 a-b and remote units 42 a-h may communicate through a different transceiver for high data rate media such as coax cable, twisted pair copper wires, free space RF or optics, or shared networks such as Ethernet, SONET, SDH, ATM and/or PDH, among others. As will be appreciated, one or more of the exemplary links, as illustrated in FIG. 2B, might be selected for coupling all of the remote units to the master units.

In some embodiments, the system 40 illustrated in FIG. 2B may be selectively and dynamically utilized as a SISO system and/or a MIMO system. For example, if the hybrid coupler 52 is not activated, the signals from the SISO BTSs 54 a-b may be transmitted to at least a portion of the remote units 42 a-h and the system may be utilized similarly to a SISO system. In this manner, each of the remote units 42 a-h communicates through at least two wireless frequencies that correspond to those used by the SISO BTSs 54 a-b. However, when the hybrid coupler 52 is selectively activated, the signals from the SISO BTSs 54 a-b may be combined with the combined MIMO output signals 68, 70 such that each remote unit 42 a-h communicates the signals from the SISO BTSs 54 a-b through at least two wireless frequencies that correspond to those used by the SISO BTSs 54 a-b and communicates both or all of the MIMO signals. Thus, selective activation of the hybrid coupler 52 results in dynamically reconfiguring the system 40 from a SISO mode of operation to a MIMO mode of operation. As such, the system 40 may be used as an indoor MIMO system that is configured to handle a WiMAX Frame Preamble and/or LTE P-SS (Primary Synchronization Signal) that either are, or optionally can be, transmitted by only one of the MIMO BTS antennas.

Thus, portions of the first and second signals 62 and 64 from the MIMO BTS 58 may be cross-coupled and combined and sent to all the remote units 42 a-h without affecting the MIMO operation thereof. For example, each remote unit 42 a-h of the system 40 may be configured to transmit both (or all) data streams from the MIMO BTS 58 and its antennas 31, 32 (e.g., the output signal 68 or the output signal 70) along with the combined SISO BTS signals 56 a-b.

FIG. 3 illustrates a schematic view of a wireless communication system 100 similar to the system 40 of FIG. 2B, but that includes a plurality of remote units 42 a-d used in an indoor environment 104. In particular, FIG. 3 illustrates that remote units 42 a-b coupled to the master unit 46 a may be positioned about the indoor environment 104 such that their signals are not substantially overlapping. Remote units 42 c-d may be similarly positioned. As such, a wireless device 90 in a portion of the environment 104 may be able to receive signals from two remote units (as illustrated in FIG. 3, wireless device 90 receives signals from remote units 42 a and 42 d). As such, MIMO spatial multiplexing may be exploited, as the wireless device 90 is capable of receiving two non-identical signals from two remote units 42 a and 42 d fed by two different master units 46 a and 46 b, respectively.

Referring to FIG. 3, by incorporating a hybrid coupler in the distributed antenna system in accordance with the invention, all of the MIMO signals (in this case, both MIMO signals) can be cross-coupled and sent to all of the remote units without affecting the MIMO operation. Each remote unit can transmit both the MIMO parallel data streams without producing inter-stream interference because there are 90° of phase shifting between them. That is, a distributed MIMO concept is split in two parallel distributed MIMO systems. The first one is “in phase”, while the second one is “90° phase shifted”. Of course, in order to exploit the MIMO spatial multiplexing, it is necessary that the wireless device 90 receive substantial power contributions from at least two of the remote units that are fed by different master units. Therefore, as such, it is desirable that the wireless device 90 receives power from more than one of the two remote units, for example 42 a, 42 d, that are coupled with different master units, in order to maintain MIMO spatial multiplexing.

One benefit of the invention is that it solves problems noted above wherein the remote units transmit signals associated with only one of the MIMO base station antennas. Similarly, performance impairments between transmitted parallel data streams that may affect the wireless unit 90 when located closer to a specific remote unit 42 a-d may be addressed, as received power levels for two signals from two remote units 42 a-d are typically similar for most locations in the indoor environment 104, thus increasing data throughput. This issue is often referred to as the “near-far problem” affecting a distributed MIMO system with remote units transmitting only a single data stream. This issue is addressed as discussed herein below using a 3 dB 90° Hybrid coupler.

Another particular benefit of the present invention is the ability to provide deployment of a MIMO system within an existing distributed antenna infrastructure that is originally implemented for a SISO system. The present invention may also operate with a selective coupling or dynamic switching between a SISO and a MIMO operation mode that is performed by a MIMO base station. Furthermore, when the MIMO base station operates in downlink spatial multiplexing mode, the invention provides the performance equalization related to the transmitted parallel data streams. That is, as noted, the 90° 3 dB hybrid coupler is used in order to solve the “near-far problem”. The inter-stream cross-coupling performed through the Hybrid Coupler acts similarly to or as a substitute for the MIMO pre-coding as specified by the 3GPP LTE standard in order to address the potential mismatch in performance between the two data streams. That is, the pre-coding provided by the invention is intended to equalize the performance (like bit error rate, error vector magnitude, etc.) of two data streams experiencing different channel conditions. In case of the “near-far problem” the two streams experience different channel path-losses. Furthermore for a proper operation of the LTE standard, it is mandatory that the pre-coding coding scheme is orthogonal so that the original symbols can be recovered at the receiver avoiding inter-stream interference. This condition is met by the 90° Hybrid Coupler input-output transfer function as discussed below in accordance with one aspect of the invention.

With reference to FIG. 8, a 90° 3 dB hybrid coupler is shown and acts as a “hardware” MIMO pre-coding circuit to compensate for possible performance impairment between the data streams (code-words) due to the “near-far problem”. The equation shown in FIG. 8 illustrates the input and output port relationship and the transfer function matrix of a 90° 3 dB hybrid coupler, respectively. As such, in accordance with an aspect of the invention, the transfer function matrix reflected in FIG. 8 can also be regarded as the MIMO pre-coding matrix of the 90° 3 dB hybrid coupler. FIG. 9 illustrates the location of the MIMO pre-coding block in a typical LTE physical channel processing stream. In accordance with an aspect of the present invention, the exploitation of a 90° 3 dB hybrid coupler into the LTE MIMO distributed antenna system as disclosed herein locates the performance of the pre-coding at the BTS antenna ports rather than in the BTS physical channel processing. Therefore the invention also represents a hardware improvement to the MIMO BTS scheduler circuitry which is responsible for pre-coding selection on the User Equipment's feedback basis.

In accordance with another aspect of the present invention, the hybrid coupler that is utilized in embodiments of the invention makes input signals orthogonal to each other. The device's reciprocity between the input ports 1, 2 and the output ports 3, 4 provides that the resulting transfer function matrix remains the same, even exchanging the input and output ports. This provides the invention with the ability to combine MIMO signals without affecting their capability to support spatial multiplexing.

FIGS. 4-6 illustrates an exemplary distributed antenna system for implementing embodiments of the invention. Focusing now on a master unit 46, FIG. 4 contains a detailed block diagram of the master unit 46. Each master unit 46 may contain from one to six radio channels (hereinafter referred to as a “path”) 110, from one to four digitally modulated optical channels 112, a controller 114, a clock generator 116, and an Ethernet switch 118.

In one embodiment, each path, such as 110 a, may be configured to handle a signal to and from SISO BTSs 54 a-b and/or MIMO BTS 58, for example. For a FDD air interface, the paths 110 a employ a combiner and a duplexer 120 to handle the uplink signal and the downlink signal. An RF downconverter 122 may amplify the received signal from the combiner/duplexer 120 to ensure that an A/D converter 124 is fully loaded. The RF downconverter 122 sets a center frequency of a band within the A/D converter pass band. The wideband A/D 124 digitizes the entire downlink band of the air interface to ensure all downlink channels are digitized. A resampler 126 converts the signal to a complex format, digitally downconverts the frequency band in some cases, decimates and filters the signal, and resamples it. This reduces the amount of data associated with a downlink signal, such as 128 a, that has to be transferred over the optical lines and synchronizes the rate of the digitized data to the optical network bit rate.

The uplink section of the radio channel 110 a sums 120 the uplink signals, such as signals 129 a-d, for its assigned band from remote units 42 coupled to the master unit 46 after they are converted to an electrical signal. The summation 130 is resampled, interpolated to change to a different data rate in some cases, and upconverted by the resampler 132 and then converted to an analog form by the D/A converter 134. The RF upconverter 136 translates the center frequency of the analog signal to the appropriate frequency for the air interface and amplifies it. The amplified signal is applied to the combiner/duplexer 120 and is routed back to the SISO BTSs 54 a-b and/or MIMO BTS 58.

In embodiments utilizing TDD air interfaces, the combiner and duplexer are replaced by a switching function 138 shown in FIG. 4 for example in radio channel 110 b and detailed in FIG. 5. While the master unit 46 is receiving the downlink signal, a RF amplifier in the RF upconverter is disabled and a shunt switch in the switching function 138 may shunt the RF amplifier to ground to further reduce leakage. During intervals when the master unit 46 is sending the uplink signal to the base station 42, the RF amplifier is enabled, the shunt switch is opened and a series switch in the switching function 138 may be opened to protect the RF downconverter from damage due to high power levels. The switch control timing 144 is determined by a master unit controller 114 from the downlink signal 128 b. Additionally, a formatter 146 may apply a data compression to reduce the redundant digital information included in the serial data stream before it is sent to the transmitter in an electro-optical transceiver 148. The compression may allow for saving bandwidth or for using a less costly transceiver with lower bit rate. The compressed serial data may be converted into an uncompressed data stream after being received on the opposite ends in the optical received of 148 by the receiver side formatter 146.

Each digitally modulated optical channel 112 a-b is composed of a formatter 146 and an electro-optical transceiver 148. On the outgoing side, the formatter 146 blocks, into time division multiplexed frames, the digitized downlink signal 128 a-b along with a customer Ethernet in Reduced Media Independent Interface (“RMII”) format 150 a-b, operation and maintenance (“O&M”) data 152 a-c and synchronization information. In other embodiments, other interfaces such as MII, RMII, GMII, SGMII, XGMII, among others may be used in place of the RMII interface. The framed data may be randomized by exclusive or'ing (XOR) it with the output of a linear feedback shift register to remove long strings of logic ones or zeros. Other known coding formats such as 8 bit/10 bit or 64 bit/66 bit coding may also be used, but may result in a decrease in efficiency in the use of the digital serial link. This digital data is then converted to a serial stream which is used to modulate an optical transmitter within the electro-optical transceiver 148. In a single fiber implementation, a wavelength division multiplexer (“WDM”) 149 may be employed to combine or split the two optical signals.

For incoming signals from the remote units 44, the electro-optical transceiver 148 converts the optical signal to an electrical signal. The formatter 146 phaselocks to the incoming bit stream and generates a bit clock that is phaselocked to the data rate and aligned with the serial data stream. The formatter 146 then converts the serial stream to a parallel digital data stream, de-randomizes it and performs frame synchronization. It then breaks out the digitized uplink signal for each band, buffers each band and routes the bands to the appropriate radio channel 110 a, 110 b, if necessary. Finally, the formatter 146 breaks out the buffers and O&M Ethernet data 152 a-c and the user Ethernet data 150 a-b and routes them to the controller 114 and the Ethernet switch 118, respectively.

The master unit controller 114 uses locally stored information and information from the O&M Ethernet data to configure and control the other blocks in the master unit 46. It also passes this information to the remote units 42 and reports status of the remote units 42 and the master unit 46 to the system controller 76. When a radio channel, such as 110 b, is assigned to a TDD air interface, the master unit controller 114 also uses the corresponding downlink signal 128 b to derive TDD switch control timing 144.

The system controller 76 generally has overall system control. The master unit controller 114 functions to configure individual modules as well as supervise individual modules. As part of the configuration and supervision functions, the master unit controller 114 is operable to determine the uplink/downlink switch timing in TDD systems by decoding the downlink signaling or acquiring it from a different source such as the time variant UL RSSI, or some base station clock signal provided from an external source. The downlink frame clock in TDMA systems may be determined and distributed by decoding the downlink signaling to allow time slot based functions such as uplink or downlink muting, uplink or downlink Received Signal Strength Indication (“RSSI”) measurements within time slots, uplink and downlink traffic analysis, etc. The master unit controller 114 may detect active channels in the RF spectrum to assist in or automatically configure the filter configuration in the resampler 126, 132. Optimal leveling of the individual signals in the resampler may also be determined by measurement of the RSSI of the various signals in the downlink RF band. A remote unit controller may perform similar tasks in the uplink of the remote unit 42.

The clock generator 116 may use a stable temperature compensated voltage controlled crystal (“TCVXO”) to generate stable clocks and reference signals 154 for master unit 46 functional blocks. Although, one of ordinary skill in the art will appreciate that other devices or crystals may also be used to generate clocking signals as long as they are capable of producing the stable clocks required by the system.

Focusing now on a remote unit 42, FIG. 5A and FIG. 5B contain a detailed block diagram of a remote unit 42 consistent with embodiments of the invention. Each unit 44 may contain from one to six radio channels 160, one or two DMOCs 162, a remote unit controller 164 and an Ethernet switch 166.

The DMOCs 162 may be designated as the downstream 168 and upstream channels 170. The downstream channel 168 is connected to a remote unit 42 that precedes this remote unit 42 in a daisy chain, if so configured. The upstream channel 170 is connected to a master unit 46 or another remote unit 42. The DMOC 162 functional blocks are similar to those in the master unit 46. Both consist of a formatter 172 and electro-optical transceiver 174. Outgoing data is buffered, formatted into frames, randomized, parallel to serial converted and used to modulate an optical transmitter in the electro-optical transceiver 174. Incoming data is converted from an optical to electrical format, bit synchronized, de-randomized, frame synchronized and converted to a parallel format. The various data types are then broken out buffered and distributed to other function blocks within the remote unit 42. In some embodiments, formatter 172 may implement compression and decompression schemes to reduce bandwidth over the digital optical link.

Radio channels in the remote unit 42 are functionally similar to those in the master unit 46. Each radio channel is configured to handle a single RF band. Unlike the master unit 46 radio channels 110, the remote unit 42 radio channels 160 are connected via a cross band coupler 176 to its antenna 44. For FDD air interfaces, the radio channels, such as radio channel 160 a, employ a duplexer 178 to split the uplink and the downlink signal. Duplexers, cross-band combiners and couplers may be optional for some embodiments of either the master unit 46 or remote units 42. In these embodiments, additional antennas may replace the duplexer 178 and cross-coupler 176 in the remote units 42. Extra cables would be required in the master unit 46. A RF downconverter 180 amplifies the received uplink signal from the antenna 44 to ensure an A/D converter 182 is fully loaded and sets the center frequency of the band within the A/D converter pass band. The wideband A/D 182 digitizes the entire uplink band of the air interface to ensure all uplink channels are digitized. A resampler 184 converts the uplink signal to a complex format, digitally downconverts the signal in some cases, decimates and filters the signal, and resamples it with a multi-rate filter bank. This reduces the amount of data that has to be transferred over the optical links and synchronizes the rate of the digitized data to the optical network bit rate. The output of the resampler 184 is added to the uplink signals 186 a from the downstream remote units 42 in summer 187. The summed uplink signal 188 a for each band is then sent to a formatter 172 in the upstream channel 170 in the DMOC 162.

The downlink signal 190 for each band (190 a, 190 b) is interpolated and frequency shifted in the resampler 192. The group delay of individual spectral components can be adjusted via filters or delay elements in the resampler 192. The signal is then converted to an analog form by the D/A converter 194. The RF upconverter 196 translates the center frequency of the analog downlink band to the appropriate frequency for the air interface and amplifies it. The amplified signal is then applied to the antenna 44 and transmitted to a wireless unit 90.

For TDD air interfaces, the duplexer 178 is replaced by the switching function 138 shown in radio channel 160 b and FIG. 5A. While the remote unit 42 is receiving the uplink, the RF power amplifier in the RF upconverter 196 is disabled and a shunt switch in the switching function 138 shunts the RF power amplifier to ground to further reduce leakage. When the remote unit 42 is transmitting the downlink signal, the RF power amplifier is enabled, the shunt switch is opened to permit the downlink signal to reach the antenna 44 and a series switch in the switching function 138 is opened to protect the RF downconverter 180 from damage due to high power levels. As with the master unit 46, the switch control timing 144 is determined by the controller 164 from the downlink signal 190 a, 190 b.

The clock generator 198 includes a voltage-controlled crystal oscillator (“VCXO”) that is phaselocked to the incoming serial data stream bit rate via a narrowband phaselocked loop (“PLL”). The VCXO output is split and is used as the frequency reference 200 for the local oscillators in each radio channel 160 a-b, the sampling clocks for the ND 182 and D/A 194 converters, and a clock for the other blocks in the remote unit 42. One of ordinary skill in the art will realize that the long term frequency accuracy should be good to ensure the local oscillators are on frequency and that the short term jitter levels should also be low to ensure that the jitter does not corrupt the A/D and D/A conversion processes. By phaselocking to the data rate of the optical link, which is derived from the stable TCVCXO in the master unit 46, the remote unit 42 does not require an expensive oven compensated oscillator or a GPS disciplining scheme to maintain long term frequency accuracy, thereby, making the more numerous remote units 42 less expensive. The use of a narrow band PLL and a crystal controlled oscillator may assist in reducing short term jitter for the ND and D/A converter clocks. Using the recovered, jitter reduced clocks 202 to re-clock the transmit data in the optical links at each remote unit 42 reduces jitter accumulation which may assist in improving A/D and D/A converter clocks in the downstream remote units 42 and may assist in reducing the bit error rate (“BER”) of the optical communication channels 162.

The remote unit controller (“RUC”) 164 uses locally stored information and information from the O&M Ethernet to configure and control the other blocks in the remote unit 42. Downstream RMII 152 d and upstream RMII 152 e may also be supplied to the formatter 172. In addition, local O&M data 206 may be configured at a local O&M terminal 204. Remote unit 42 also passes this information to the up and downstream remote units 42 and/or master unit 46. The RUC 164 additionally uses the appropriate downlink signal to derive TDD switch control timing 144 when required.

In an alternate embodiment of the radio channel 160 c utilized in a remote unit 42, the radio channel 160 c may also employ digital pre-distortion to linearize the power amplifier. This embodiment of the radio channel 160 c in a remote unit 42 is shown in the block diagrams of FIG. 6. In this embodiment, a third signal path may be added to one or more radio channels 160 c. The third path couples off the downlink signal after power amplification and digitizes it. The signal from the antenna 44 is received in an RF downconverter 208, which amplifies the received signal to ensure an A/D converter 210 is fully loaded and sets the center frequency of the band within the A/D converter pass band. The wideband A/D 210 digitizes the entire uplink band of the air interface to ensure all uplink channels are digitized. The digitized signal is compared to a delayed version of the downlink signal in the digital pre-distortion unit 212 and the difference is used to adaptively adjust the gain and the phase of the signal prior to D/A conversion to correct for non-linearity in the power amplifier.

While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, a distributed antenna system consistent with embodiments of the invention may have more or fewer remote units 42, master units 46, summing circuits 48, RF combination networks 50, hybrid couplers 52, SISO BTSs 54, MIMO BTSs 58 and/or system controllers 76 than those illustrated. In particular, each MIMO BTS 58 may include more or fewer output ports 62 and/or 64.

Additionally, each master unit 46 may be connected to more or fewer remote units 42 than those illustrated. As such, a plurality of remote units 42 may be connected to each master unit 46 through two links and/or along a single link. Alternatively, each remote unit 42 may be connected to a master unit 46 through a dedicated link. In some embodiments, up to six remote units 42 may be connected in series from a master unit 46. As such, remote units 42 may be positioned to optimize coverage within a coverage area.

Furthermore, system 40 and/or 100 may not include summing circuits 48 a-48 b. As such, the master unit 46 a may combine the combined SISO BTS signal 56 a and first output signal 68, while the master unit 46 b may combine the combined SISO BTS signal 56 b and second output signal 70. Additionally, the system 40 may also not include RF combination network 50. As such, the master unit 46 a may combine one or more signals from the SISO BTSs 54 and the first output signal 68, while the master unit 46 b may combine one or more signals from the SISO BTSs 54 and the second output signal 70.

Moreover, and in some embodiments, the master unit controller 114 may measure a pilot signal strength of CDMA or Orthogonal Frequency-Division Multiplexing (“OFDM”) signals to properly set the level of the downlink signals, as the RSSI can vary at different capacity loading. The pilot signals generally remain constant with a configured ratio between pilot level and a maximum composite for full loading, the required headroom for the signals may be maintained. The master unit controller 114 may also measure and supervise the signal quality of the provided downlink channels. In case of signal degradation, an alarm may be set and the operator can focus on a base station (e.g., SISO or MIMO BTS) without having to troubleshoot the entire system 40 and/or 100.

In some embodiments, the master unit controller 114 determines the amount of channels for a narrowband base station standard such as Global System for Mobile communications (“GSM”). Together with the measurement of the Broadcast Control Channel (“BCCH”), which is constant in power, the proper headroom that is required for a multichannel subband may be determined and overdrive or underdrive conditions may be avoided. In other embodiments, the master unit controller 114 monitors the crest factor of a transmitted spectrum in the presence of multiple channels. The crest factor may provide input to the leveling of the transmit power or the power back-off of particular gain stages of the system. The configured headroom is generally higher than the measured crest factor to avoid signal degradation due to clipping or distortion. In addition, a crest factor reduction mechanism may be employed in the resampler in some of the embodiments to reduce the crest factor and make more efficient use of the RF power amplifier in the remote unit 42 or assist in reducing the number of required bits per sample that need to be transmitted over the link.

As illustrated in FIGS. 7A and 7B, the invention provides benefits in regard to the Uplink path of a MIMO communication system. Both WiMAX and LTE wireless standards encompass Uplink MIMO features. In particular the “Uplink Collaborative MIMO” is implemented in Mobile WiMAX, while “Uplink Multi-User MIMO” is the term adopted in LTE for indicating the same technique. The peculiarity of this MIMO scheme is to increase the total Uplink sector capacity by reusing time/frequency resources allocated to the different UEs (User Equipments) or mobile devices, rather than to boost the data rate per single user as for Downlink Single-User MIMO (Spatial Multiplexing).

FIG. 7A shows a MIMO BTS 300 in an outdoor scenario, which can coordinate the data reception from two different mobile devices A and B, equipped with a single transmitter (Tx) antenna each, and then allocate the same time/frequency resources to them. The decoding of their respective data streams is performed by the BTS through the same signal processing as for a Single-User MIMO situation. That is, the two data streams belonging to spatially separated users, rather than to a single user with two co-located Tx antennas, are spatially multiplexed. As a consequence the saved time/frequency resources can be allocated to more users in order to increase the total Uplink sector capacity. Finally the MIMO transmission might benefit from the fact that the two transmitters are largely separated leading to a consequent increase of the probability to have uncorrelated radio channels which is one important requirement for successful MIMO operation.

FIG. 7B highlights such a potential benefit in an indoor system. FIG. 7B illustrates a schematic view of at least a portion of a wireless communication system 220 somewhat similar to the system 100 of FIG. 3, but that does not show the plurality of SISO BTSs 54 a-b, the RF combination network 50, and the summing circuits 48 a-b. With respect to FIG. 3, like reference numerals are utilized in FIG. 7B where applicable. The system 220 of FIG. 7 includes a plurality of master units 46 a-b that provide signals to respective remote units 42 a-b with a single Rx antenna each and positioned in respective portions 208 a-b of an indoor environment 224. In particular the indoor environment example is illustrated as two rooms 228 a-b separated by a wall 230.

As illustrated in FIG. 7B, the respective portions 228 a-b of the indoor environment 224 are somewhat electromagnetically isolated (e.g., low levels of a signal sent from a wireless device 232 (device A or B) in one portion 228 detected by the remote unit 42 of another portion 228). In specific embodiments, the respective portions 228 a-b are separated by the partition or wall 230. This illustration shows the case of good Uplink power isolation between the different remote unites 42 a-b and the related mobile devices 232 a and 232 b. Therefore the Uplink multi-user MIMO feature operates desirably because at the BTS antenna ports, the mutual interference of the signals from the two devices 232 a-b only depends on the isolation provided by the indoor radio planning. Although isolation will be determined by the deployment of the remote units and the location of the users and mobile devices, indoor scenarios offer good isolation due to the presence of multiple walls and floors. Also, the hybrid coupler of the invention doesn't affect the BTS MIMO decoder since the signals from the mobile devices 232 a-b are orthogonally cross-coupled to the BTS antenna ports thus avoiding their mutual interference. Therefore in another aspect of the invention, with two fully isolated groups of users served by two remote units connected to different Master Units, the Uplink Multi-User MIMO feature can achieve a complete reuse of the time/frequency resources of the BTS. As a consequence the number of users manageable in the Uplink path by the MIMO BTS would be increased and possibly doubled.

It will be appreciated that such an aspect of the invention might be in contrast to the feature discussed herein of maintaining a certain degree of signal coverage overlapping between Remote Units as requested by the Downlink Single-User MIMO when implemented through DAS. Therefore, for realizing both such advantages, a tradeoff would have to be considered and managed to balance the benefits of both the MIMO features. In this context the same 90° 3 dB Hybrid Coupler can be exploited both in Downlink and in Uplink paths of an indoor DAS for MIMO signals.

Thus, each remote unit 42 a-b provides signals to, and receives signals from, respective wireless devices 232 a-b that are present within those respective portions 228 a-b. One benefit of this arrangement as noted is that uplink collaborative MIMO (for WiMAX) and/or uplink multi-user MIMO (for LTE) may be used to increase the total uplink capacity by reusing the time and/or frequency resources associated with the different wireless devices 232 a-b.

The invention in its broader aspects is not limited to the specific details representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the applicants' general inventive concept. For example, the system 10 of FIG. 2A, the system 40 of FIG. 2B, the system 100 of FIG. 3, and/or the system 220 of FIG. 7 may be configured with an extension unit (not shown) disposed between a master unit 46 and its corresponding remote units 42. The extension unit may provide additional links for coupling a master unit 46 to additional remote units 42 and/or the extension unit may extend the range of coupling between a master unit 46 and remote units 42. Moreover, the system 10 of FIG. 2A, the system 40 of FIG. 2B, the system 100 of FIG. 3, and/or the system 220 of FIG. 7 may be configured with more or fewer remote units 12 or 42, master units 16 or 46, SISO BTSs 20 or 54, MIMO BTSs 30 or 58, system controllers 22 or 76, summing circuits 48, RF combination networks 50, and/or hybrid couplers 52, as well as support more or fewer wireless devices 26, 90, and/or 232 consistent with embodiments of the invention. Similarly, the system 10 of FIG. 2A, the system 40 of FIG. 2B, the system 100 of FIG. 3, and/or the system 220 of FIG. 7 may include a MIMO BTS 30 or 58 with more or fewer antennas 31 and/or 32, a hybrid coupler 52 with more or fewer ports, as well as a master unit 16 or 46 configured with more or fewer inputs or outputs consistent with embodiments of the invention.

Additionally, it will be appreciated that the indoor environments 104 and 224 of FIGS. 3 and 7B, respectively, are merely included to show operation of embodiments of the invention therewith, and that embodiments of the invention may be used with outdoor environments without departing from the scope of the applicants' general inventive concept. Moreover, one of ordinary skill in the art will appreciate that system 220 of FIG. 7B may include SISO BTSs 54 a-b as well as the RF combination network 50 and summing circuits 48 a-b consistent with alternative embodiments of the invention.

Furthermore, in some embodiments, the indoor environment 224 of FIG. 7B will be configured in other ways than just including partition 230. As such, the respective wireless devices 232 a-b may be isolated in other ways.

FIGS. 10-12 illustrate further alternative systems for incorporating embodiments of the invention within a distributed antenna system. Specifically, system 250 illustrated in FIG. 10 is an interleaved distributed antenna system, incorporating MIMO signals, wherein the deployment of the system is implemented utilizing a combination of optical fibers, as well as an RF passive distribution network, for the purposes of signal distribution. Specifically, optical fiber is used as a link for handling traffic between the master units and remote units, while an RF distribution network is implemented between the remote units and one or more passive antennas coupled to the remote units. To that end, system 250 illustrates a particular distributed antenna system that incorporates a MIMO BTS 252 for handling signals from one or more antennas 253, 254. Such a system may take advantage of the present invention. Although the embodiments illustrated in FIGS. 10-12 incorporate two antennas and two MIMO signals (n=2), as noted herein, a MIMO system of the invention may incorporate additional antenna elements and signals as well. Therefore, the invention is not limited to the illustrated number of different MIMO antennas or MIMO channels.

The MIMO signals 255, 256, from respective MIMO antennas 253, 254 of the MIMO BTS, are delivered to master unit 260 in a suitable fashion, such as using an RF network (e.g., Coaxial cables), for distribution throughout the remote components of the system. Similarly, uplink signals from wireless devices are delivered to the MIMO BTS through the master unit 260. Although a single master unit is illustrated in the illustrated figures and discussed herein, the system 250 might utilize one or more master units.

To that end, the MIMO signals are appropriately processed and delivered to one or more remote units 262, 264 for further distribution of those signals to wireless devices and equipment, such as cellular phones. A suitable signal link, such as a fiber link 266, 268, is incorporated between the master unit 260 and the various respective remote units. The master unit 260 and remote units 262, 264 can handle the multiple MIMO signals or channels 255, 256 in an appropriate fashion over the fiber links 266, 268. For example, frequency conversion might be implemented wherein the multiple MIMO channels (two channels in the illustrated example) are converted in frequency so that their integrity is maintained over the fiber links 266, 268. Alternatively, the fiber links 266, 268 might incorporate multiple fibers wherein each fiber carriers a separate MIMO channel signal (Antenna 1/Channel 1 or Antenna 2/Channel 2) to maintain the integrity of MIMO signals between the master unit 260 and the multiple remote units 262, 264. In still another embodiment, wavelength division multiplexing (WDM) might be implemented on the fiber links 266, 268 between the master and remote units to maintain the integrity of the MIMO channels.

In the system 250 illustrated in FIG. 10, the various signals, including the MIMO signals, are then passively distributed from the remote units in an RF distribution network out to passive antenna elements 270 a-270 d. For example, the remote units 262, 264 might be coupled to the antennas 270 a-270 d through suitable RF links, which might implement RF cables, such as coaxial cables, and suitable power splitters for delivering the signals to multiple passive antennas 270 a-270 d. In the system 250, the MIMO channels are still segregated, and thus, each particular antenna unit 270 a-270 d would only handle one of the MIMO channels.

Referring now to FIG. 11, in accordance with one embodiment of the invention, a hybrid coupler circuit is implemented in the system 251. Specifically, a hybrid coupler circuit is implemented between each of the remote units 262 and 264 and the respective passive antennas 270 a-270 d. As illustrated in FIG. 11, the hybrid coupler circuits 274, which may be 90°, 3 dB hybrid RF couplers as discussed herein, are shown situated between the various remote units and antennas. As illustrated in FIG. 11, each antenna 270 a-270 d then handles a portion of each of the MIMO signals in accordance with the invention for communication with a wireless device, such as wireless device 280.

As discussed above, the hybrid coupler circuits are configured to receive the first MIMO signal (Antenna 1/Channel 1) and the second MIMO signal (Antenna 2/Channel 2) from the respective remote unit at respective first and second ports. Output signals are then provided at output ports of the couplers, and the output signals include portions of the first and second MIMO signals. The output signals, with the combined MIMO signal portions, are then delivered to the various antenna elements 270 a and 270 d and broadcast appropriately to wireless devices in the signal vicinity. In that way, each antenna in the indoor environment handles both or all MIMO channels as illustrated.

FIG. 12 illustrates another alternative embodiment of the invention, wherein a hybrid coupler circuit 274 is implemented within an optical fiber and RF distribution system 258, similar to that as illustrated in FIG. 10. Rather than incorporating a hybrid coupler circuit with each of the remote units, a hybrid coupler circuit 274 is incorporated between the MIMO BTS 252 and the master unit 260. The coupler circuit combines the multiple MIMO channels on output ports as discussed and the various output ports are coupled with the master unit 260. The combined MIMO signals at the master unit are then appropriately directed to the remote units 262, 264 utilizing fiber, and then further distributed through an RF link or passive distribution network to the antennas 270 a-270 d, as discussed above. Similar to the embodiment in FIG. 11, each antenna in the indoor environment handles both or all MIMO channels as illustrated.

FIGS. 13-15 illustrate other alternative embodiments of the invention, incorporating radiating elements alternative to the antenna radiating elements in other figures. In one embodiment of the invention, the radiating elements may be in the form of radiating cable elements or sections, as illustrated in FIGS. 13-15. Such radiating cable elements may be useful in any confined space, such as a train tunnel, or other tunnel, and are arranged in an elongated fashion in the space for transceiving wireless systems with mobile devices and other user equipment. The invention can be implemented with legacy SISO remote units that may be installed within an existing space, as shown in FIG. 13. Alternatively, as illustrated in FIGS. 14 and 15, dedicated MIMO remote units having a plurality of outputs might be utilized, and thereby might incorporate a plurality of radiating cable elements with each remote unit.

Turning to FIG. 13, system 300 includes one or more remote units 302 to provide coverage in a service area, such as an enclosed tunnel area 304. In particular, system 300 includes one or more remote units 302 a-302 d communicating with a MIMO signal source 307. For example, the remote units 302 may be existing legacy SISO remote units that were previously installed in the space of tunnel 304, and utilized with a SISO distributed antenna system (DAS). The invention provides a way for providing MIMO services using legacy SISO systems for providing cost savings in various installations. Each remote unit 302 a-d includes at least one radiating element 320, 321 for transmitting and receiving RF signals through the remote unit. Each of the remote units 302 are coupled to a master unit 306 that interfaces with a source of a plurality of MIMO signals and operates as a MIMO master unit for the system. The master unit 306 communicates, or is coupled with, a signal source of the MIMO signals, such as MIMO BTS 307, which provides a plurality of MIMO signals including Channel 1 and Channel 2 MIMO signals, for example. The FIG. 13 illustrates just two MIMO signals, but as noted herein, additional MIMO signals might be handled as well, in accordance with the principles of the invention. The MIMO signals 308, 310 from the MIMO base station or other signal source are presented through a hybrid coupler 312 in the embodiment of FIG. 13.

In one aspect of the present invention, a hybrid coupler 312 is coupled between the source, MIMO BTS 307, and master unit 306 to cross-couple all MIMO signals to the master unit 306, to then be forwarded to each of the SISO remote units 302. Therefore, each of the remote units 302 transmits signals that contain some portion of all of the MIMO data streams. In the embodiment of FIGS. 13-15, the radiating elements used at the remote units are radiating cable elements 320, 321, 350, 352. Such radiating cable elements are suitable for use in long enclosed areas, such as tunnel 304, and the radiating cable elements may run a portion of the length of the tunnel 304 for suitable coverage.

The hybrid coupler 312 is configured to receive at least two MIMO signals from the MIMO BTS on respective input ports 322, and provide output signals on appropriate output ports 324. In the illustrated embodiment of FIG. 13, combined MIMO signals from the MIMO BTS are provided at the output ports 324. Each output signal 326, 328 includes at least a portion of the first MIMO signal (CH1), and at least a portion of the second MIMO signal (CH2). In the hybrid coupler circuit 312, the portion of the first MIMO signal 308 (e.g., CH1) or the portion of the second signal 310 (e.g., CH2) presented at one of the input ports 322, is phase shifted with respect to the first signal (CH1) and/or second signal (CH2) presented at the respective other of first and second coupler input ports 308, 310. In particular, the hybrid coupler 312 is disposed between MIMO BTS 307 and master station 306 such that the hybrid coupler 312 is configured to receive the first and second MIMO signals 308, 310 from the MIMO BTS 307, including first MIMO channel signal (CH1), and second MIMO channel signal (CH2). In turn, the hybrid coupler 312 combines a portion of the first MIMO signal CH1 with a phase-shifted portion of the second MIMO signal CH2 and outputs that first combined output signal (CH1+jCH2) at a first output port as signal 326. Coupler 312 also combines a portion of the second MIMO signal CH2 with a phase-shifted portion of the first MIMO signal CH1 and outputs that second combined output signal (CH2+jCH1) at a second output port as signal 328. In one exemplary embodiment, the hybrid coupler 312 is a 90° 3 dB coupler (also referred to as a “quadrature” coupler).

As illustrated in FIG. 13, the MIMO master unit 306 processes the signals (CH1+jCH2) and (CH2+jCH1) and is coupled to the plurality of remote units 302, through an appropriate signal transport medium or link 309, such as an RF cable link or optical fiber link. The link could be similar to other such links, as described herein, such as links 80, 82, 84, 86, for handling appropriate uplink and downlink signals between the master unit and remote units. Each of the remote units 302 is then coupled with an appropriate radiating element, such as a radiating cable element 320, 321. As illustrated, the master unit 306 directs at least one of the combined MIMO signals, such as (CH1+jCH2) or (CH2+jCH1) to a particular remote unit 302 through link 309. As such, each of the remote units will handle one of the combined signals, which has a portion of each MIMO channel signal CH1, CH2. Referring to FIG. 13, remote unit 302 b and respective radiating cable element 320 are shown handling CH1+jCH2 as a combined MIMO signal. Alternatively, remote unit 302 c and radiating cable element 321 are shown handling CH2+jCH1 as a combined MIMO signal. Therefore, in an embodiment of the invention incorporating SISO remote units, the plurality of combined MIMO signals provided by the master unit 306 are transmitted by different remote units and radiating cable elements. In that way, each of the combined MIMO signals will be transmitted into the tunnel area 304 handled by the distributed system 300 so that a communication link is provided to various mobile devices 330 within the tunnel area 304. As noted herein, the embodiment of the invention as illustrated in FIG. 13, solves problems noted herein regarding remote units that transmit only one of the MIMO channel signals. The system 300 also provides various other benefits, as noted herein.

FIG. 13A illustrates another embodiment of the invention implementing SISO remote units similar to FIG. 13. However, the master units implemented are also SISO master units. As such, they might be part of a legacy DAS system. Each of the individual SISO master units 311 a, 311 b are provided with one of the outputs from coupler 312. Similar reference numerals are utilized as applicable between FIG. 13 and FIG. 13A.

Referring to FIG. 13A, coupler 312 directs the respective output signals from the output ports to the SISO master units 311 a, 311 b. As noted herein, there may be a greater number of master units than just the two illustrated in FIG. 13A in accordance with the invention.

Each of the master units is coupled to one or more remote units, as illustrated in FIG. 13. For example, master unit 311 a is coupled with remote units 302 a, 302 b. Similarly, master unit 311 b is coupled with remote units 302 c and 302 d. As shown, each of the remote units 302 a-302 d will be coupled with an appropriate radiating cable element 320, 321. The respective remote units, in radiating cable elements, will transmit and receive the signal associated with each of the SISO master units. That is, some of the remote units will handle one of the combined MIMO signals, and the other set of remote units will handle the other combined MIMO signal, as illustrated in FIG. 13A.

In FIGS. 13 and 13A, with different sets of remote units handling respective combined MIMO signals, the radiating cable elements might be arranged so as to be implemented for multi-user MIMO. To that end, one set of the radiating elements and remote units might be implemented in one particular area, such as one segment of a tunnel, and the other radiating cable elements and remote units might be implemented in another area or another segment of the tunnel. In that way, the signal conditions for multi-user MIMO features might be provided.

FIG. 14 illustrates an embodiment of the invention incorporating a coupler circuit in accordance with the invention, wherein the various remote units are MIMO remote units capable of handling multiple MIMO channel signals, and coupled with a plurality of radiating cable elements. In the embodiment of FIG. 14, similar to FIG. 11, a hybrid coupler circuit is implemented between each of the remote units and the appropriate radiating elements, such as radiating cable elements. FIG. 14 resembles FIG. 13 in various ways, and thus, common elements are given similar reference numerals.

Referring now to FIG. 14, system 340 provides a distributed system which handles MIMO signals 308, 310 from a MIMO signal source, such as a MIMO BTS 307, that are directed to a MIMO master unit 306. The MIMO master unit provides the multiple MIMO signals, including the Channel 1 MIMO signal (CH1) and the Channel 2 MIMO signal (CH2), to each of a plurality of MIMO remote units 342, through an appropriate signal transport medium/link 309, such as an RF cable link or optical fiber link. Each of the remote units 342 a-d directs the MIMO signals CH1, CH2 out of respective output ports shown as CH1, CH2, in FIG. 14, to respective first and second input ports 344 of a coupler circuit 346 coupled with each remote unit. In accordance with the invention, the coupler circuit 346 combines the first MIMO channel signal CH1 and the second MIMO channel signal as noted herein to provide output signals on appropriate output ports 348 of the coupler circuit. The output signals are combined signals that include both MIMO channel signals. For example, one output signal of coupler circuit 346 is reflected as the combined MIMO signal CH1+jCH2. The other signal is reflected as CH2+jCH1. As noted, the output signals include at least a portion of one MIMO signal, and at least a portion of the other MIMO signal that is phase shifted.

In accordance with the embodiment illustrated in FIG. 14, a radiating cable element 350 or 352 is coupled with a respective coupler circuit output port. For example, a first radiating cable element 350 handles the combined MIMO signal CH1+jCH2. Alternatively, a second and different radiating cable element 352 handles combined MIMO signal CH2+jCH1, such signals are transmitted in the appropriate enclosed space, such as tunnel 304, for MIMO communications with mobile device 330. As noted, the coupler circuits might be 90° 3 dB hybrid RF couplers.

FIG. 15 illustrates a further embodiment of the invention somewhat similar to the arrangement illustrated in FIG. 12, and implementing radiating elements that are radiating cable elements, to act as an antenna for transmitting and receiving signals in the distributed system. Because of the similarities of elements between FIGS. 13, 14, and 15, like reference numerals are implemented in FIG. 15 as well. As illustrated in FIG. 15, a MIMO BTS 307 provides a MIMO master unit 306 with the MIMO channel signals, through a coupler circuit 312. The MIMO master unit 306 thus handles combined MIMO signals that incorporate portions of each of the MIMO channel signals, as noted herein. One distinction in the embodiment of FIG. 15 with respect to the system shown in FIG. 13 is that the distributed system 360 illustrated in FIG. 15 incorporates MIMO remote units that have a plurality of output ports for coupling to a plurality of appropriate radiating elements, such as radiating cable elements 350, 352. The MIMO master unit 306 is linked with each of the various MIMO remote units 362 a-d through an appropriate signal transmission medium/link. The MIMO remote units 362 handle both of the combined output signals and provide the combined MIMO channel signals at respective output ports 364, as shown to a plurality of radiating cable elements. For example, one of the output ports of a remote unit 362 would include the combined signal CH1+jCH2, and would direct that signal to element 350. Another of the output ports would handle the combined signal CH2+jCH1, which is directed to element 352. While the remote units 362 handle the plurality of combined output signals, each of those combined MIMO signals is directed to an appropriate radiating element, such as elements 350, 352, for communications with device 330 in the tunnel environment 304. In that way, FIG. 15 provides various of the advantages and benefits of the invention, in accordance with the principles discussed herein. Combining of the signals is handled prior to delivery of those signals to the master unit in FIG. 15, rather than between the remote unit and the respective radiating elements, as illustrated in FIG. 14.

Other modifications will be apparent to one of ordinary skill in the art. Therefore, the invention lies in the claims hereinafter appended. Furthermore, while embodiments of the invention has been illustrated by a description of the various embodiments and the examples, and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants' general inventive concept. 

What is claimed is:
 1. A system comprising: a multiple-input and multiple-output (MIMO) signal source operable for providing MIMO signals including at least a first MIMO signal and at least a second MIMO signal; a plurality of remote units communicating with the MIMO signal source and configured to receive the first MIMO signal and second MIMO signal, each remote unit providing the MIMO signals at respective output ports; at least one radiating cable element for radiating signals into a space; a hybrid coupler coupled between the output ports of a remote unit and the cable radiating element and configured to receive the first MIMO signal and the second MIMO signal from the remote unit on respective input ports and to provide an output signal on at least one output port, the output signal including at least a portion of the first MIMO signal and at least a portion of the second MIMO signal; the radiating cable element coupled with the at least one output port.
 2. The system of claim 1, wherein the remote unit communicates with the master unit over at least one of an optical link or an RF link.
 3. The system of claim 1, wherein the hybrid coupler is a 90° 3 dB hybrid coupler.
 4. The system of claim 1, wherein the coupler is further configured to output a plurality of output signals on a plurality of output ports, each of the plurality of output signals including at least a portion of the first MIMO signal and at least a portion of the second MIMO signal, a radiating cable element being coupled with each output port.
 5. The system of claim 1, wherein at least one of the portions of the first and second MIMO signals making up the output signal is phase shifted.
 6. The system of claim 1, wherein at least one of the portion of the first MIMO signal or the portion of the second MIMO signal is phase shifted orthogonally in relation to the respective first MIMO signal and second MIMO signal.
 7. The system of claim 4 wherein at least one output signal on an output port of the coupler includes a portion of the first MIMO signal and a phase shifted version of the second MIMO signal and a first radiating cable element is coupled to transmit that respective signal, and another output signal on an output port of the coupler includes a portion of the second MIMO signal and a phase shifted version of the first MIMO signal and a second radiating cable element is coupled to transmit that respective signal.
 8. A system comprising: at least one master unit configured for handling signals; a hybrid coupler coupled with the master unit to receive a first MIMO signal and a second MIMO signal from a signal source on respective input ports and configured to provide an output signal on at least one output port, the output signal including at least a portion of the first MIMO signal and at least a portion of the second MIMO signal, the master unit coupled to receive the coupler output signal; a plurality of remote units communicating with the master unit and configured to receive the coupler output signal from the master unit; at least one radiating cable element for radiating signals into a space and coupled with at least one remote unit for radiating the output signal.
 9. The system of claim 8 wherein the remote units communicate with the master unit over at least one of an optical link or an RF link.
 10. The system of claim 8, wherein the hybrid coupler is a 90° 3 dB hybrid coupler.
 11. The system of claim 8, wherein the coupler is further configured to output a plurality of output signals on a plurality of output ports, each of the plurality of output signals including at least a portion of the first MIMO signal and at least a portion of the second MIMO signal, a remote unit and respective radiating cable element coupled to receive and radiate each output signal.
 12. The system of claim 8, wherein at least one of the portions of the first and second MIMO signals making up the output signal is phase shifted.
 13. The system of claim 11 wherein the remote units are SISO remote units coupled with a radiating cable element, each of the plurality of output signals transmitted by a different remote unit.
 14. The system of claim 11 wherein the remote units are MIMO remote units coupled with a plurality of radiating cable elements, each of the plurality of output signals transmitted by the same remote unit.
 15. The system of claim 14 wherein each of the plurality of output signals is transmitted by different radiating cable elements.
 16. A system comprising: a multiple-input and multiple-output (MIMO) signal source operable for providing MIMO signals including at least a first MIMO signal and at least a second MIMO signal; a hybrid coupler coupled with the signal source and configured to receive both the first MIMO signal and the second MIMO signal on respective input ports and configured to provide output signals on respective output ports with each output signal including at least a portion of both the first and second MIMO signals; an output signal of one output port including at least a portion of the first MIMO signal and at least a portion of the second MIMO signal that is phase shifted orthogonally in relation to the second MIMO signal; an output signal of another output port including at least a portion of the second MIMO signal and at least a portion of the first MIMO signal that is phase shifted orthogonally in relation to the first MIMO signal; a plurality of radiating cable elements operably coupled to receive the output signals, at least one radiating cable element receiving the signal of the one output port and at least one radiating cable element receiving the signal of the another output port for radiating the signals into a space.
 17. The system of claim 16 further comprising a master unit coupled with the MIMO signal source, the coupler positioned between the signal source and the master unit, the coupler delivering the output signals to the master unit to be delivered to the radiating cable elements.
 18. The system of claim 16 further comprising a master unit coupled with the MIMO signal source and a plurality of remote units coupled with the master unit, the radiating cable elements operably coupled with the remote units to receive the output signals.
 19. The system of claim 18 wherein the coupler is positioned between a remote unit and at least one radiating cable element coupled thereto, the radiating cable element operably coupled to receive an output signal from the coupler.
 20. The system of claim 19 wherein the coupler is positioned between a remote unit and a plurality of radiating cable elements coupled thereto, the radiating cable elements operably coupled to receive the output signals from the coupler. 